Fix test : do not consider deleted migrations

* Change way to detect deleted migrations (diff-filter of 'git diff' command)

tests/test_project_fixtures/test_deleted_migration_project/git/COMMIT_EDITMSG

@@ -1 +1,16 @@
 Delete file and insert new
+
+# Please enter the commit message for your changes. Lines starting
+# with '#' will be ignored, and an empty message aborts the commit.
+#
+# Date:      Thu Jan 18 23:27:37 2018 +0100
+#
+# On branch master
+# Changes to be committed:
+#	deleted:    test_app/migrations/0002_new_not_null_field.py
+#	new file:   test_app/migrations/0003_new_nullable_field.py
+#	modified:   test_app/models.py
+#
+# Untracked files:
+#	git/
+#

tests/test_project_fixtures/test_deleted_migration_project/git/logs/HEAD

@@ -1,3 +1,4 @@
 0000000000000000000000000000000000000000 0395cbee60f2ea1b97f42ebdd43991c8f5d8d76e David-Wobrock <david.wobrock@gmail.com> 1516314144 +0100	commit (initial): Project init
 0395cbee60f2ea1b97f42ebdd43991c8f5d8d76e 154ecf6119325cc3b1f3f5a4e709bfbd61a4a4ba David-Wobrock <david.wobrock@gmail.com> 1516314322 +0100	commit: Add backward incompatible migration
 154ecf6119325cc3b1f3f5a4e709bfbd61a4a4ba be2fae887ee7ecd2d1e6371af1a865c17414bde9 David-Wobrock <david.wobrock@gmail.com> 1516314457 +0100	commit: Delete file and insert new
+be2fae887ee7ecd2d1e6371af1a865c17414bde9 4bdc4ba9e7695893423be1b54f3101d6ccbd11c7 David-Wobrock <david.wobrock@gmail.com> 1516459290 +0100	commit (amend): Delete file and insert new

tests/test_project_fixtures/test_deleted_migration_project/git/logs/refs/heads/master

@@ -1,3 +1,4 @@
 0000000000000000000000000000000000000000 0395cbee60f2ea1b97f42ebdd43991c8f5d8d76e David-Wobrock <david.wobrock@gmail.com> 1516314144 +0100	commit (initial): Project init
 0395cbee60f2ea1b97f42ebdd43991c8f5d8d76e 154ecf6119325cc3b1f3f5a4e709bfbd61a4a4ba David-Wobrock <david.wobrock@gmail.com> 1516314322 +0100	commit: Add backward incompatible migration
 154ecf6119325cc3b1f3f5a4e709bfbd61a4a4ba be2fae887ee7ecd2d1e6371af1a865c17414bde9 David-Wobrock <david.wobrock@gmail.com> 1516314457 +0100	commit: Delete file and insert new
+be2fae887ee7ecd2d1e6371af1a865c17414bde9 4bdc4ba9e7695893423be1b54f3101d6ccbd11c7 David-Wobrock <david.wobrock@gmail.com> 1516459290 +0100	commit (amend): Delete file and insert new

tests/test_project_fixtures/test_deleted_migration_project/git/objects/4b/330bb1c7154afb23f170218d170dae079c437f

@@ -0,0 +1,2 @@
+x
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+Â0E]íW¼±	RAÈ …‚ƒ›{Hš×´ò’H’¢ŸocÄÉñ^Îá^E^A]ïW}ðôU:ã™V0Ú›	¬×H±(:’1±\2;çº:k7‰~DÒÀß0;¹„C›Û2ü&¬fïÂù$þ“ZIñcuƒ/M*B‘0¦ßÑf&–E+‚Й4ðÝv_ž<4Si

tests/test_project_fixtures/test_deleted_migration_project/git/refs/heads/master

@@ -1 +1 @@
-be2fae887ee7ecd2d1e6371af1a865c17414bde9
+4bdc4ba9e7695893423be1b54f3101d6ccbd11c7

tests/test_project_fixtures/test_deleted_migration_project/test_app/migrations/0003_new_nullable_field.py

@@ -1,18 +1,15 @@
-# Generated by Django 2.0.1 on 2018-01-18 22:26
-
-from django.db import migrations, models
+from django.db import migrations
+from django.db import models
 
 
 class Migration(migrations.Migration):
 
-    dependencies = [
-        ('test_app', '0001_initial'),
-    ]
+    dependencies = [('test_app', '0001_initial'),]
 
     operations = [
         migrations.AddField(
             model_name='a',
-            name='new_nullable_field',
-            field=models.IntegerField(null=True),
+            name='new_char_nullable_test_field',
+            field=models.CharField(max_length=30, null=True),
         ),
     ]

tests/test_project_fixtures/test_deleted_migration_project/test_app/models.py

@@ -3,4 +3,4 @@ from django.db import models
 class A(models.Model):
 	null_field = models.IntegerField(null=True)
 	new_not_null_field = models.IntegerField(null=False)
-	new_nullable_field = models.IntegerField(null=True)
+	new_char_nullable_test_field = models.CharField(max_length=30, null=True)